Many current wireline and LWD logging tools measure formation and/or fluid properties with a relatively shallow depth of-investigation (DOI). Logging data acquired in these shallow DOIs can be significantly affected by the borehole mud filtrate invasion. For example, the sensitive volume of typical Nuclear Magnetic Resonance (NMR) logging instruments ranges from about 1 inch (2.54 cm) to about 4 inches (10.2 cm). NMR sensitive volumes are typically in a flushed zone or invaded zone. As the result, NMR-based fluid typing and saturation estimate may be contaminated by filtrates. For example, the oil-based mud-filtrate (OBMF) is miscible to the native reservoir crude oil. When OBMF invasion occurs, measurements of properties such as hydrocarbon saturation and fluid viscosity made by an NMR tool are affected not only by the native oil but also by OBMF. Nuclear, acoustic, shallow resistivity logging results, as well as fluid sampling measurements, can all be affected by filtrate invasion.
The problem is illustrated in FIG. 2. In FIG. 2 the wellbore 22 is shown with the NMR probe 42 disposed therein. A series of radial positions RP1, RP2, . . . , RPn are shown as concentrically surrounding the wellbore 22. Each radial position RPx occupies a portion of the surrounding formation 26, and represents a depth of investigation (DOI). The wellbore 22 traverses a gas zone 60. A portion of the gas zone 60 suffers from invasion, which is depicted as an invasion zone 29. Also as shown in FIG. 2, the sensitive volume 58 includes areas of the gas zone 60, and the invasion zone 29.
Although the sensitive volume 58 is depicted as cylindrical or circular, this is not to be construed as a limitation. The method of the present disclosure is not limited to cylindrical or a portion of the cylindrical shape. For example, the sensitive volume 58 of the MREX™ instrument is not strictly circular. The sensitive volume does not have the same width of the ring (thinner on the sides so it is like a new crescent). In short, the sensitive volume 58 may include a variety of shapes and other geometric properties.
U.S. Pat. No. 7,565,246 to Fang et al., having the same assignee as the present disclosure, teaches a method for determining fluid saturation in a formation at a plurality of radial depths near a wellbore. The method includes: obtaining multi-frequency nuclear magnetic resonance (NMR) response data for the formation; and processing the data to determine simultaneously the fluid saturation at each radial depth. This enables identification of the invaded zone.
The problem of fluid invasion is also present in other shallow measurements. These include formation fluid sampling devices. For example, U.S. Pat. No. 6,871,713 to Meister et al., having the same assignee as the present disclosure and the contents of which are incorporated herein by reference, discloses a formation testing and sampling tool that enables sampling of parameters of formation fluids near a sampling port. Care has to be taken to make sure that the fluid sample is not contaminated with borehole fluid.
An active method of mitigating the effect of fluid invasion is disclosed in U.S. Pat. No. 7,497,256 to DiFoggio et al., having the same assignee as the present application and the contents of which are incorporated herein by reference. DiFoggio teaches the use of a barrier tube that substantially surrounds the probe and is extended into the formation. Another active method is disclosed in U.S. patent application Ser. No. 12/857,091 of Herrick, having the same assignee as the present application and the contents of which are incorporated herein by reference. Herrick teaches the use of a cathode associated with the BHA during drilling operations that is configured to produce a static electric field in the earth formation and inhibit a flow of water from the borehole into the earth formation.
The present disclosure addresses the problem of reducing the invasion of borehole fluids into the formation. Inhibiting the invasion of borehole fluids makes the method of the present disclosure useful not only for NMR measurements but also for other shallow measurements.